The present invention relates to a method of testing a semiconductor pressure sensor and more specifically to a method of testing the pressure sensitivity of a diaphragm type semiconductor pressure sensor typical of a semiconductor pressure sensor fitted to the tip of a catheter for medical use.
When a mechanical stress is applied to a semiconductor made of a silicon or the like, its resistance changes greatly because of the piezoelectric resistance effect and the perception of this fact has led to the development of a semiconductor pressure sensor. The process of making such a semiconductor pressure sensor comprises the steps of forming strain gauge resistors formed by diffusion of impurity ions on the surface layer of a silicon single crystal, assembling four of the strain gauge resistors into a Wheatstone bridge, forming a recess in the back surface of the silicon single crystal to use a thin portion thereof as a diaphragm, and disposing pads in suitable places on the front surface except for the diaphragm. When pressure is applied to the semiconductor pressure sensor, the diaphragm is deformed and the resistance value of the strain gauge resistor changes to a greater extent because of the piezoelectric resistance effect, so that a bridge output proportional to the pressure is obtainable.
The aforesaid semiconductor pressure sensor is extremely small in size and, particularly in the case of a semiconductor pressure sensor for medical use, a plurality of semiconductor pressure sensors are fitted on the tip of a catheter and inserted into a body. Accordingly, even in a semiconductor pressure sensor incorporating peripheral circuits such as a temperature compensating circuit, a pressure sensitivity compensating circuit and the like, a side of a chip should be about 1 mm or smaller in length.
It is therefore extremely difficult to measure the bridge output by applying pressure to the diaphragm from the surface of the semiconductor pressure sensor and simultaneously make the electrode of the semiconductor pressure sensor contact the measuring probe.
The conventional testing method for a semiconductor pressure sensor under the wafer process comprises mounting a wafer of the semiconductor pressure sensor on a wafer stage and making the electrodes built in the surface of the wafer contact a measuring probe without applying pressure so that only electrical measurement is effected.
In the aforesaid method, however, no testing is made when pressure is being applied to the diaphragm. Further, the thickness of the diaphragm formed in each semiconductor pressure sensor through the process of ion implantation, diffusion, epitaxial growth, etching etc. lacks uniformity, so that the degree of the deformation of each diaphragm against the actually applied pressure is slightly different from each other. As a result, accurate measurement cannot be carried out. Since the chip size is small, it is practically impossible to measure the pressure sensitivity by applying pressure to each chip after the wafer is cut out in order to prevent such variation in deformation. Accordingly, the sensor has to be used with allowance for the aforesaid variation. Although it is considered that the pressure sensitivity is measured after the chip is cut out of the water, those chips that do not have a desired pressure sensitivity, together with the chip, must be discarded. It simply means a waste of labor and cost.